Because free will is an inherent good and overriding free will is therefore at least somewhat evil. A person who is forced to do good things is not himself evil, but neither is he good. His capacity to choose good has been destroyed.
Do you think that slavery is good if the slaveowner only makes his slaves do good things? What if he frees his slaves and one of them becomes a criminal that causes suffering? Is his decision to free them now an evil decision?
Also, I would add that suffering is not an inherent evil. It can be the result of evil, but it can also exist for other reasons. Reducing suffering is good, but when other evil is committed to reduce suffering, that action may not be good. Every person on the earth will suffer at some point in the future. If reducing suffering was an inherent good, murdering people might be justified for the simple reason that killing them will prevent their future suffering. Of course, their death might cause others to suffer additionally, but it might not. And in the hypothetical, the additional sufferers could also be killed. Balefiring an entire city would likely reduce a great deal of suffering without so many loose ends to suffer so long as whole families are taken together.
To be clear, I realize that these aren't easy questions and that the answers are also not black and white. That's why it's interesting for a series of thousands of pages to explore these issues. But to say that it makes no sense for free will to be recognized for its inherent value and good is an oversimplification that deserves pushback and exploration.